1. Field of the Invention
The present invention relates to a detection circuit subjected to temperature compensation for eliminating erroneous detection caused by a temperature coefficient when magnitude of an alternating current signal is detected using a comparator.
2. Description of the Related Art
Conventionally, for example, in connection with a base unit and a handset in a cordless telephone, at the time of telephone conversation using the handset, the farther the handset moves away from the base unit, the more difficult it is to hear voice, and when a human voice reaches the same level as that of the surrounding noise, the handset is in a so-called out-of-service area. Under this condition, battery power is only consumed, and therefore the handset is controlled to stop outputting voice.
For this reason, a noise detection circuit is provided in the handset to extract, using a bandpass filter, a signal having a frequency around 20 kHz, which is the highest in an audible range, from audio signals having frequency of several kHz. The extracted signal as a noise signal is rectified and smoothed, the resultant is converted into a direct current voltage, and the direct current voltage is compared with a reference voltage. As a result, when the level of the direct current voltage is larger than that of the reference voltage, it is determined that the amount of noise is large, and a voice call is interrupted.
In such a noise detection circuit, when a noise signal is extracted, and voltage obtained by DC-converting the extracted signal is compared with the reference voltage to detect magnitude of the noise, a detection circuit subjected to temperature compensation must be used to prevent occurrence of erroneous noise determination.
As a conventional circuit subjected to temperature compensation, there is disclosed a constant current circuit as shown in, for example, Patent Document 1 (Japanese Patent Laid-Open No. 2001-142552).
Patent Document 1 describes the constant current circuit in which temperature coefficient of an output current Io is set to 0, but does not describe at all that magnitude of a signal level with respect to a reference level is detected by use of a comparator.
In order to achieve a temperature-independent noise detection circuit, there is need to reduce to zero a temperature coefficient of the direct current voltage as one input of the comparator and a temperature coefficient of the reference voltage as the other input of the comparator. For achieving this, used is at least one of two types of current sources including a current source (hereinafter referred to as a VT proportional current source) that is proportional to voltage proportional to temperature (hereinafter referred to as VT), and a current source (hereinafter referred to as a current source with no temperature coefficient) that does not depend on temperature generated by use of the VT proportional current source. Here, a case is considered in which two types of current sources are used in performing temperature compensation.
In a general circuit design, used is, rather than a system where current is generated in each circuit block in order to reduce the circuit scale, a system where current is generated in a current source block at one time and the generated current is distributed to each circuit block therefrom. In this case, if a plurality of current sources are needed, a plurality of distribution passages are required, thus raising a problem that a wiring region is increased. For this reason, there is considered a detection circuit subjected to temperature compensation with one current source. Accordingly, for achieving temperature compensation with one current source, the VT proportional current source is generally used to newly form a circuit that generates temperature independent current.
However, when two voltages with no temperature coefficient are generated and then compared with each other as mentioned above, a problem occurs in which the number of elements is increased due to newly prepared current generation circuit that does not depend on temperature.